Hanna Instruments HI 904 - Manual (Page 158)

10
TITRATION THEORY
of reaction decreases and titration endpoint become increasingly difficult to reach. If the pH
exceeds 8, side reactions begin to occur between iodine and hydroxide or methylate ions,
changing the titration stoichiometry.
While solvents not containing alcohols can be used for Karl Fischer analysis, they also have
an effect on reaction stoichiometry. When alcohols are not present, the reaction resembles
the Bunsen reaction stoichiometry where the consumption ratio of water to iodine is 2:1. In
solvents containing higher alcohols, uneven ratios can be observed due to the relative abilities
of higher alcohols to form the sulfite ester that reacts with water. Issues resulting from
solvent-induced variation in stoichiometry are not typically encountered during routine analysis
for two reasons. First, titrant standardization and sample analysis are carried out in the same
titration medium and under the same conditions, effectively compensating for any variation
in reaction behavior. Second, most Karl Fischer reagent system are formulated to support
standard KF reaction stoichiometry.
2.2.1.2 Volumetric Karl Fischer Titrations
In volumetric Karl Fischer titrations, the iodine for the Karl Fischer reaction is introduced via
the titrant. This method is suitable for higher water contents: 100 ppm – 100%. The exact
strength of the titrant (titer) is determined by standardization with a water standard. The
other reaction components (sulfur dioxide, base, alcohol) can either be introduced by the
titrant (one-component system) or by the solvent (two-component system). One-component
reagent systems can utilize a custom solvent or solvent mixture since all of the Karl Fischer
reaction components are in the titrant. However, one-component reagents do not have very
stable titers, do not have a long shelf life, and suffer slower titration speeds. Two-component
reagent systems have the advantage of fast titration speeds, stable titers, and long shelf
lives, but choice of solvent is limited to commercial availability.
2.2.1.3 Coulometric Karl Fischer Titrations
In coulometric Karl Fischer titrations, the iodine for the Karl Fischer reaction is generated
electrolytically inside the titration vessel, as opposed to introducing iodine via a titrant solution.
This method is suitable for lower water contents: 1 ppm – 5%. The generator consists of
two electrodes: an anode and a cathode. The reaction that occurs at each can be summarized
as follows:
Anode: 2 I¯ I
2
+ 2 e¯
Cathode: 2 RN-H + + 2 e¯ H
2
+ 2 RN
The iodine that is generated at the anode reacts with the water from the sample according to
the Karl Fischer reaction. The amount of water that is reacted during a titration can be
calculated based on the total charge that has passed through the generator. According to the
Karl Fischer reaction (in protic solvents), 1 mole of water is titrated by 1 mole of iodine.
According to the anodic reaction above, 1 mole of iodine is generated with 2 moles of
electrons. Faraday’s Constant states that 1 mole of electrons equates to 96485 coulombs (C)
of electricity. Therefore, 96485 coulombs will cause 0.5 moles of water to be titrated, or 1
coulomb equals 93.36 µg of water:
1 C ·
(
1 mol e¯
)
·
(
1 mol I
2
)
·
(
1 mol H
2
O
)
·
(
18.015 g H
2
O
)
= 9.336 · 10 -5 g H
2
O
= 93.36 µg H
2
O
96485 C 2 mol e¯ 1 mol I
2
1 mol H
2
O